Process for identification marking clear plastic articles

A process for imparting a permanent identification mark to an optically clear plastic article without adversely affecting its visible light optical properties, said process comprising the steps of:

• (a) mixing a solid ultraviolet light activatable fluorescent dye with a liquid carrier material to produce a marking composition;
• (b) mixing a solid sublimeable inert diluent with said dye and said liquid carrier;
• (c) causing at least a portion of said carrier to evaporate so that said dye and said solid diluent form a paste in said liquid carrier;
• (d) applying a pattern of said paste marking composition to a surface of the optically clear plastic article;
• (e) evaporating a remaining portion of said carrier material from the surface of the article to leave a residue of solid dye and solid diluent in the pattern on said surface, the amount of dye on said surface being sufficient to induce a readily detectable fluorescent response when irradiated with light from a U.V. lamp, yet insufficient to produce a visually detectable response in the presence of sunlight;
• and
• (f) heating the surface of the article to a selected temperature below the degradation point of the plastic for a sufficient amount of time to transfer the dye into a surface layer of said article to result in a fade resistant mark and to sublime said solid diluent.